The present invention relates to a normal temperature-operating electrolyte fuel cell for use in such applications as a portable power source, an electric vehicle and a home powder source system.
The normal temperature-operating polymer electrolyte fuel cell generates electricity by electrochemically reacting a fuel gas such as hydrogen and an oxidizer gas such as oxygen. The heat, which simultaneously comes out in the reaction, is also made good use of.
The basic construction composing a polymer electrolyte fuel cell is configured as in the following, for example. A polymer electrolyte film is made of a hydrogen ion conductive resin such as a fluorocarbon resin having a sulfo group. A catalytic layer substantially made of a carbon powder with a platinum group metal catalyst carried thereon is formed closely on each surfaces of the polymer electrolyte film. Furthermore, a gas-permeable and electrically conductive electrode layer is provided directly on the outer surface of each catalytic layer. On the outer surface of the electrode layer, an electrically conductive separator is provided to mechanically clamp the assembly composed of the electrode layers and the polymer electrolyte film and to electrically connect the assembly with the adjoining assembly in series. A groove-shaped channel to evenly supply the gas to the electrode layer is formed on the surface of the separator facing the electrode layer. The fuel gas is supplied to one of the pair of the electrode layers, and the oxidizer gas is supplied to the other. Hereafter described an arrangement in which hydrogen is used as the fuel gas and oxygen as the oxidizer gas.
A hydrogen gas supplied from outside is taken in the electrode layer on the hydrogen gas supplying side or the anode while passing along the surface of the electrode layer. The hydrogen gas taken in the electrode layer diffuses and passes through the electrode layer, then reaches the catalytic layer. When the hydrogen gas flows inside the catalytic layer and reaches a region where the polymer electrolyte coexists, an electrochemical reaction is caused between the hydrogen gas and the polymer electrolyte. The hydrogen gas is ionized by the reaction, and the generated hydrogen ions are taken into the polymer electrolyte film.
On the other hand, an oxygen gas supplied from outside is taken in the electrode layer on the oxygen gas supplying side or the cathode while passing along the surface of the electrode layer. The oxygen gas diffuses and passes through the electrode layer, then reaches the catalytic layer on the cathode side. Then, the oxygen gas turns into water vapor by reacting with hydrogen ions drifting from the anode through the electrolyte film. In this reaction, the electrons migrate from the anode to the cathode through an external load connected to the fuel cell. This migration of the electrons is utilized as an electric power. This electrochemical reaction between hydrogen and oxygen further produces heat. Therefore, a cooling water is circulated through the inside of the fuel cell to keep the temperature of the fuel cell low, and the warmed water is utilized as a thermal energy.
The polymer electrolyte fuel cell is usually operated in a temperature range from a room temperature to about 80.degree. C. Therefore, the water vapor generated by the electrochemical reaction on the catalytic layer on the cathode side mostly condenses into dew around the catalytic layer. When the condensed water remains around the catalytic layer, the oxygen gas is prevented from reaching the reaction area or the catalytic layer, and hence the cell performance is lowered. On the anode side, meanwhile, no water is generated. However, in a case the water generated on the cathode side permeates through the polymer electrolyte film and penetrates into the catalytic layer, or in a case the water vapor which is previously mixed into the fuel gas to prevent the electrolyte film from drying up condenses into dew and remains on the catalytic layer, the supply of hydrogen to the reaction area is blocked and the cell performance is dropped.
To solve the problems, various countermeasures have been proposed and tried, which include a water-repellent finish of the electrode layers to maintain the reaction area or the catalytic layers in a good wet condition, and an increased flow velocity of the gas flowing along the surface of the electrode layers so as to remove an excessive water sticking to the electrode layers.
However, the employment of the water repellent finished electrode layer causes a problem of hardly removing the water, resulting in a decreased cell performance, under some operating conditions as, for example, a high current density output operation which generates a large quantity of water or a low gas flow velocity operation. In some serious cases, the gas channels on the surfaces of the electrode layers are clogged, with the output of the cell dropping to zero.